Thank you for your reply, please see my comments below.
@vecnar so you have a smoke sensor with a normally open relay contact, and the contact closes when smoke detection is triggered. As I understand the contact is dry ( potential free).
Smoke sensors have an interconnect wire that is used to communicate with other sensors (no voltage on it during normal operation), it uses itself and negative ac wire. There is 9 volt dc only when there is alarm (between interconnect wire and negative ac wire). So i assume what you wrote is correct.
Your alarm system requires normally closed circuit and triggers on open circuit. (ignoring the resistor and tamper for now).
Correct. I will use zone only for this purpose and it will only operate when alarm is set. I will not need any resistors or tamper on this zone.
You what to connect the two together, no mysensors stuff in between.
Correct, just some type of relay/board that wouldn't rely on ac power as both smoke sensors and alarm have battery backup, smoke sensors 9v DC and alarm has 12v DC.
Did I understand correctly?
Yes. Just not sure if i correctly answered your first question regarding dry contact.
I am trying to achieve the same thing as Edward Cheung the only difference is my alarm zone is normally closed where his is normally opened and not exactly sure about zone current that can be used from alarm panel, below is a spec sheet of my house alarm.